Generally, control of steam turbines comprises partial arc admission and full arc admission depending on whether all nozzles are active during operation. They have different advantages in respective application, which is known per se to those skilled in the art. Quite often, a partial arc admission steam turbine is required to be converted to be a full arc admission steam turbine, such as retrofitting exiting partial arc admission steam turbine and adapt to applications where full arc admission is desired.
Conventionally, a partial arc admission steam turbine comprises a plurality of nozzle boxes, at least two, which are assembled to be a complete circle, and which are communicated correspondingly with a plurality of nozzle plates, generally one nozzle box for each nozzle plate. To achieve full arc admission, the nozzle boxes could be removed. However, on some machines where the nozzle boxes are welded to the turbine casing, the removing requires significant site work and a long outage for this turbine. This approach also increases the duty of the existing outer casing of the turbine which makes it necessary to re-qualify the design hence imposing difficulty for implementation.
As another approach, the whole outer casing of the turbine may be replaced with new full arc admission casing. However, this solution is extremely expensive and requires site pipework welding and hence long outage.
As another approach, the pipework between the control valves and the casing can be joined to create the effect of full arc admission. However, this requires requalification of the pipework, which may involve complete upgrade of the system.
In view of this, there exists the need of a solution that may be used to convert existing partial arc steam turbine into full arc admission in a cost effective, operable, and reliable manner.